[0001] The present invention is concerned with joining a workpiece to a substrate and is
particularly concerned with joining an integrated circuit chip to a substrate. The
present invention is especially concerned with soldering in a reducing atmosphere.
[0002] During the preparation of integrated circuit modules, input/output (I/O) pins are
inserted into the substrate so as to protrude from one surface of the substrate, known
as the bottom side, for insertion into circuit cards or circuit boards. The pins also
protrude slightly from the other surface referred to as the top side so as to contact
the circuitry on the top side which, in turn, connects to the integrated circuit chip
or chips which are subsequently connected to the substrate or carrier. When the pins
are to be connected to the substrate circuitry by a soldering process, solder flux
compositions have been applied to the pins.
[0003] Solder flux compositions are employed to remove oxides from the pins and to prevent
the pins from oxidizing when subjected to elevated temperature for soldering and
thereby serving to maintain the electrical conductivity of the pins.
[0004] Once the solder is applied, any flux composition or residue therefrom, (e.g., unpolymerized
species) remaining on the pins and substrate must be removed to provide as clean
a substrate as possible. This involves a cleaning step which normally employs an organic
composi tion such as N-methyl pyrollidone or a halogenated hydrocarbon such as perchloroethylene
in the case of rosin-base flux compositions.
[0005] In addition, integrated circuit chips are attached to the integrated substrate or
carrier by applying solder to preselected areas on the substrate which is generally
referred to in the art as the "chip pad area". Such areas can be defined, for instance,
by providing preselected exposed areas of a metal such as copper which will accept
the solder. Moreover, a flux composition would normally be applied to the substrate
to facilitate the application of the solder to the chip pad area. After solder is
applied to the chip pad area, any flux and/or flux residue must be removed prior to
attaching the integrated circuit chip so as to provide as clean a module as possible.
[0006] The present invention makes it possible to eliminate the liquid flux compositions
previously required and, in turn, the cleaning steps to remove the flux and/or flux
residue. This, in turn, eliminates the precautions necessary for pollution abatement.
[0007] Another aspect of the present invention is that the only by-products produced are
water, small amounts of carbon dioxide and some hydrocarbon gases.
[0008] The present invention is concerned with a process for joining a workpiece to a substrate.
According to preferred aspects, the process provides, through soldering, an electrical
connection between both the pins and the substrate circuitry, and the substrate circuitry
and the workpiece. The process includes applying solder to the area of the substrate
where the workpiece is to be subsequently joined and/or to the workpiece. A dihydric
or polyhydric alcohol or ether derivative is applied in the area of the substrate
where the workpiece is to be subsequently joined. This area may or may not be devoid
of solder. A workpiece, which can contain solder, is then placed over this area and
in contact with at least a portion of the dihydric and/or polyhydric alcohol and/or
ether derivative thereof on the substrate. The resulting assembly is heated in a reducing
atmosphere to thereby join the workpiece to the substrate.
[0009] The entire joining or soldering process, with the exception of the dihydric and/or
polyhydric alcohol and/or ether derivative thereof, is a dry process. Dry processes
are desirable since such include fewer process steps, less contamination, and better
dimensional control when compared to the wet processes employed.
Detailed Description of Embodiment
[0010] The substrates to which the workpiece is joined are preferably those substrates commonly
used in the preparation of integrated circuit modules or carriers such as ceramic
substrates. A ceramic is a product or material manufactured by the action of heat
on earthy raw materials. Preferred ceramic substrates include aluminum oxides, silicon
oxides, and silicates such as aluminum silicate. The substrate preferably includes
preformed holes through which input/output pins are inserted so as to protrude from
one surface of the substrate, known as the bottom side, for insertion into the circuit
cards or circuit boards. The pins also protrude slightly from the other surface, referred
to as the top side, so as to contact the circuitry on the top side which, in turn,
connects to the integrated circuit chip or chips mounted on the back side of the substrate.
The pins of the module can be any of the input/output pins well-known in the art and
need not be discussed here in any great detail. Examples of suitable pins are gold-plated
pins, copper-plated pins, solid copper pins, nickel alloys, copper alloys, and tin-plated
pins.
[0011] If desired, after the pinning operation, the assembly can be cleaned to remove oils
and fingerprints by immersion in an aqueous detergent composition. According to preferred
aspects of the present invention, any oxides formed on the pins and substrate can
be removed by subjecting the assembly to elevated temperatures in a reducing atmosphere
such as in a hydrogen atmosphere containing less than 10 ppm of oxygen, and preferably
less than 1 ppm of oxygen, and most preferably no oxygen. It is also preferred that
the atmosphere in the furnace contain less than 50 ppm of water and most preferably,
no water. The temperature in the hydrogen atmosphere is usually about 300°C to about
500°C and preferably about 350°C to about 450°C. The cleaning can be carried out in
a furnace either batch-wise or continuously employing a conveyor belt. A suitable
conveyor belt type of container exhibiting the desired hydrogen atmosphere is the
platinum catalytic container disclosed in European Patent Application No. 0 180 131,
disclosure of which is incor porated herein by reference. In addition, an automatic
batch-type furnace which is suitable is disclosed in U.S. Patent 4,568,277 disclosure
of which is incorporated herein by reference.
[0012] The flow rate of the gas in the furnace is usually about 850...1420 dm³ (about 30
to about 50 standard cubic feet) per hour and preferably about 1070...1190 dm³ (about
38 to about 42 standard cubic feet) per hour; an example of which is about 1130 dm³
(40 standard cubic feet) per hour.
[0013] The assembly is exposed to the reducing atmosphere for at least about 1 to about
3 minutes at temperatures of at least about 350°C and preferably about 2 to about
2.5 minutes at temperatures of at least about 350°C. In a conveyor type of furnace
of about 5.50 m (216 inches) long, the conveyor speed is about 12...38 cm (5...15
inches) per minute and preferably about 28 cm (11 inches) per minute.
[0014] Solder is applied to the pin heads which are slightly protruding from the top side
of the substrate. One convenient form of applying the solder is in the form of rings
having dimensions suitable for placing around the diameter of the pin heads. Solder
can, but does not have to be applied to the area of the substrate in which the chip
or chips are to be subsequently joined. Instead of or in addition to the solder on
the substrate where the chip(s) are to be joined, the solder can be applied to the
surface of the chip(s) which are to confront the substrate.
[0015] Examples of suitable solders are tin-lead solders. Preferred tin-lead solders contain
about 3% to about 63% by weight of tin and, correspondingly, about 97% to about 37%
by weight of lead. For a discussion of the configuration in which the pins and chips
are joined to a substrate, see U.S. Patent 3,229,240 and U.S. Patent 4,233,620, disclosures
of which are incorporated herein by reference. Typical examples of such tin-lead solders
are 3% tin-97% lead; 5% tin-95% lead; 10% tin-90% lead; 63% tin-37% lead; and 60%
tin-40% lead.
[0016] Next, a thin layer of a dihydric alcohol and/or polyhydric alcohol and/or ether derivative
thereof is applied to the substrate on at least a portion of that area where the chip
or chips or workpiece are to be joined to the substrate.
[0017] The alcohols and ether derivatives employed are generally saturated aliphatic alcohols
or ether thereof having 2 to 6 hydroxyl groups and containing up to about 20 carbon
atoms and preferably 10 carbon atoms or less. In addition, it is preferred that the
alcohols and ether derivatives vaporize and/or decompose at temperatures of less than
about 350°C and preferably less than about 300°C.
[0018] Examples of dihydric alcohols, polyhydric alcohols, and ether derivatives are 1,2-ethanediol;
1,2-propanediol; 1,3-propanediol; 1,3-butanediol; 1,4-butanediol; 1,5-pentanediol;
2,5-hexanediol; 1,6-hexanediol, glycerol; pentaerythritol; diethylene glycol; triethylene
glycol; tetraethylene glycol; dipropylene glycol; and polyethylene glycols and polypropylene
glycols having molecular weights of about 200-6,000. Mixtures of alcohols and/or mixtures
of ether derivatives thereof can be used when desired. The preferred alcohols are
the propylene glycols with 1,2-propanediol being the most preferred. In general, the
preferred alcohols and/or ethers employed exhibit viscosities at room temperature
of at least about 50 centipoises. Moreover, it is preferred that the alcohols and/or
ethers, when employed in the present invention, are not combined with any compounds
having atoms other than carbon, oxygen, and/or hydrogen and are not combined with
materials which would leave solid residue on the substrate after the soldering step,
and most preferably not combined with any other materials.
[0019] The alcohol and/or ether derivatives thereof function as fugitive adhesives in holding
the chip(s) in place on the substrate during the soldering processing.
[0020] The amount of alcohol and/or ether derivative applied need only be a very thin layer
and, accordingly, such can be applied with an eye dropper. In fact, less than one
drop of such is sufficient for the purposes of the present invention. Of course, greater
amounts can be employed without adversely affecting the process of the present invention.
[0021] The circuit chip or chips are then applied to the substrate in contact with the alcohol
and/or ether derivative thereof. The alcohol and/or ether derivative thereof holds
the chip(s) in place in a manner similar to the function of normal flux compositions
containing rosin or abietic acid. However, unlike normal flux compositions, the alcohol
and/or ether derivative does not provide the chemical fluxing action. This, in accordance
with the present invention, is provided by gas in the reducing atmosphere.
[0022] The assembly is then heated in a reducing atmosphere in order to join the chip to
the substrate and to simultaneously cause tinning of the pin heads. This step assures
an electrical connection between the pins and the circuitry. The conditions of the
reducing atmosphere are the same as those discussed hereinabove for the step of removing
oxide from the pins and substrate. The oxygen content of the reducing atmosphere is
less than 10 ppm, preferably less than 1 ppm, and most preferably entirely free from
oxygen. The temperature employed is about 300°C to about 500°C and preferably about
350°C to about 450°C. The dwell time of exposure of the assembly to these conditions
is about 1 to about 5 minutes. In a conveyor type furnace of about 5.50 m (216 inches)
long, the conveyor speed is about 12...38 cm (5...15) inches per minute and preferably
about 28 cm (11 inches) per minute.
[0023] The heating in the reducing atmosphere results in tinning and soldering of the chips
and also results in the conversion of the alcohol and/or ether derivative thereof
to water, CO₂, and some saturated hydrocarbon gases including propane and smaller
amounts of ethane and methane.
[0024] Next, the assembly is then processed according to conventional methods to form the
final module.
[0025] As can be appreciated, the process of the present invention can be considered a fluxless
process for soldering whereby the only by-products are gases and certain hydrocarbons
which can be readily removed by burning off with excess hydrogen. Since no residue
is left on the assembly after the soldering, the normal cleaning operations involving
organic solvents is eliminated.
[0026] Also, the present invention makes it possible to both join the chips to the substrate
and to solder the heads of the pins simultaneously in a hydrogen atmosphere. Of course,
if desired, the steps can be done separately.
[0027] It has been observed that solder joints of the chips to the substrate have reduced
occurrence of voids as compared to those obtained using conventional prior art flux
compositions described above. Also, the expected life of the solder joints is increased
as compared to using conventional prior art flux compositions.
[0028] Although the above process has been described with respect to joining chips to a
substrate for convenience, it is understood that the process of the present invention
is applicable to joining workpieces in general to a substrate.
1. A process for joining a workpiece to a substrate which comprises
a. applying solder to the area of said substrate where the workpiece is to be joined,
or to said workpiece, or to both;
b. applying to at least a portion of the area of said substrate where the workpiece
is to be joined, a material selected from the group of dihydric alcohols, polyhydric
alcohols, ether derivatives thereof, or mixtures thereof;
c. positioning a workpiece on said substrate in contact with at least a portion of
said material;
d. heating the resulting assembly in a reducing atmosphere; and
e. thereby joining said workpiece to said substrate.
2. The process of claim 1 wherein said workpiece is an integrated circuit chip.
3. The process of claim 1 wherein said substrate is a ceramic substrate.
4. The process of claim 1 wherein said reducing atmosphere is a hydrogen atmosphere
wherein the amount of oxygen is less than 10 ppm.
5. The process of claim 1 wherein the temperature in the heating step is about 300°C
to about 500°C.
6. The process of claim 1 wherein said heating is carried out for about 1 to about
3 minutes.
7. The process of claim 1 wherein said reducing atmosphere is 100% hydrogen.
8. The process of claim 1 wherein said material is a saturated aliphatic alcohol or
ether thereof containing 2 to 6 hydroxyl groups.
9. The process of claim 8 wherein said material contains 20 carbon atoms or less.
10. The process of claim 8 which further includes applying solder to the pin heads
which protrude from the top side of the substrate and simultaneously joining the chip
to the substrate and soldering the pin heads.
11. The process of claim 1 wherein said material is a 1,3-propanediol.
12. The process of claim 1 wherein said material is 1,2-propanediol.
13. The process of claim 1 which further comprises inserting input/output pins into
the substrate prior to applying the solder.
14. The process of claim 13 wherein after insertion of the pins and prior to applying
the solder, the assembly is subjected to heating in a reducing atmosphere in order
to remove oxide.